Altered Scale

The altered scale is a common scale for creating tension over dominant chords. It includes all of the "spicy" notes that push towards the resolving tonic chord, including the root, ♭9, ♯9, 3, ♭5 or ♯11, ♭13, and ♭7 of the chord.

You can think of the altered scale as being built off of the 7th mode of the melodic minor scale; for example, if you play a melodic minor scale starting on the 7th scale degree, you are playing the altered scale.

In context, the altered scale for G7 is comes from A♭ melodic minor (G is the 7th scale degree).